In a CMOS type image sensor, an image pixel portion is constructed by disposing a number of pixels on a semiconductor chip in a two-dimensional matrix manner; and each pixel includes a photoelectric conversion element such as a photodiode or the like, which generates signal charges in response to the amount of received light, and a gate circuit composed of a plurality of MOS transistors for converting the signal charges generated by those photoelectric conversion elements into electrical signals to be read at a predetermined timing. Also, in the vicinity of the image pixel portion there are provided: a signal processing circuit which performs signal processing such as CDS (Correlated Double Sampling) or the like with respect to a signal from each pixel, a vertical/horizontal scanner circuit for driving a gate circuit of each pixel to read signals of each pixel in a predetermined order, and a shutter scanner circuit for clicking the electronic shutter.
In a conventional CMOS type image sensor, as pixel arrangement of an image pixel portion 10 is schematically shown in FIG. 11, a number of pixels 12 are disposed in the image pixel portion 10 in a two-dimensional matrix manner, and an electronic shutter row 14 which resets a signal charge of each pixel 12 and a read selecting row 16 which reads a pixel signal are selected at an interval of a predetermined number of rows in between and then sequentially shifted in the direction indicated by the arrow A.
At each pixel 12 in the electronic shutter row 14, an operation of discarding photoelectrons (charge) accumulated in a photodiode of each pixel 12 is performed.
Also, at each pixel 12 in the read selecting row 16, an operation of converting photoelectrons accumulated in a photodiode of each pixel 12 into an electrical signal to be output to a signal processing circuit through an output signal line is performed.
Accordingly, the output signal of each pixel 12 becomes a signal whose level corresponds to the amount of photoelectrons accumulated in the photodiode between the times when each pixel 12 is reset in the electronic shutter row 14 and that is selected for being read in the read selecting row 16.
Consequently, sensitivity can be adjusted by changing the interval between the electronic shutter row 14 and the read selecting row 16 to alter a period of time in which photoelectrons are accumulated.
Conventional electronic shutters function as described above.
However, in the above-mentioned method, when a bright part and a dark part coexist in an image to be picked up, there is a problem in which sensitivity is not sufficient at the dark part if the shutter time is adjusted corresponding to that for the bright part, and a photodiode at the bright part gets saturated and completely whitened if the shutter time is adjusted corresponding to the dark part, namely a problem of narrow dynamic range has been noted.
Accordingly, as a method for widening the dynamic range there has recently been proposed, for example, the method disclosed in Japanese laid-open patent publication No. 2001-177775.
In the above method a signal of a photodiode is read in a plurality of divided times in an image-signal readout selecting row.
However, in this method, due to the miniaturization of image pickup devices and decrease in the number of saturated electrons in a photodiode caused by employing lower voltages in recent years, one transfer can never satisfy electric potential sufficient in a floating diffusion (hereinafter called FD) portion, so that signals can not sufficiently be obtained after the first time even if the reading is carried out a plurality of times; therefore it is impossible to increase dynamic range in this case.
Also, in this case, since the reading has to be consecutively executed twice or more in one selected row, it is difficult to speed up the shifting of pixel rows.
Also, in order to remove fixed-pattern noise of each pixel, if CDS processing which takes the difference between signal levels at the time of a reset and at the time of photoelectrons being accumulated is executed, it is necessary to perform the resetting operation twice and to perform the operation of accumulating photoelectrons twice for reading two pixel signals in one selected row, so that reset level detection and photoelectron accumulation level detection have to be executed four times in total.
Also, since the above-described method is of combining a plurality of pixel signals that have been read in a plurality of divided times, circuits for the combination is required at a subsequent stage, thereby causing problems in which circuits are made to be large in size, and there occurs dispersion in combined portions and among circuits for the combination, each of which is provided in each pixel row.